Increasingly dire warnings of the dangers of climate change by the world's scientific community combined with greater public awareness and concern over the issue has prompted increased momentum towards global regulation aimed at reducing man-made greenhouse gas (GHGs) emissions, most notably carbon dioxide. Ultimately, a significant cut in North American and global CO2 emissions will require reductions from the electricity production sector, the single largest source of CO2 worldwide. According to the International Energy Agency's (IEA) GHG Program, as of 2006 there were nearly 5,000 fossil fuel power plants worldwide generating nearly 11 billion tons of CO2, representing nearly 40% of total global anthropogenic CO2 emissions. Of these emissions from the power generation sector, 61% were from coal fired plants. Although the long-term agenda advocated by governments is replacement of fossil fuel generation by renewables, growing energy demand, combined with the enormous dependence on fossil generation in the near to medium term dictates that this fossil base remain operational. Thus, to implement an effective GHG reduction system will require that the CO2 emissions generated by this sector be mitigated, with carbon capture and storage (CCS) providing one of the best known solutions.
The CCS process removes CO2 from a CO2 containing flue gas, enables production of a highly concentrated CO2 gas stream which is compressed and transported to a sequestration site. This site may be a depleted oil field or a saline aquifer. Sequestration in ocean and mineral carbonation are two alternate ways to sequester that are in the research phase. Captured CO2 can also be used for enhanced oil recovery.
Current technologies for CO2 capture are based primarily on the use of amines solutions which are circulated through two main distinct units: an absorption tower coupled to a desorption (or stripping) tower.
A very significant barrier to adoption of carbon capture technology on large scale is cost of capture. Conventional CO2 capture with available technology, based primarily on the use of amine solvents, is an energy intensive process that involves heating the solvent to high temperature to strip the CO2 (and regenerate the solvent) for underground sequestration. The conventional use of amines involves an associated capture cost of approximately US $60 per ton of CO2 (IPCC), which represents approximately 80% of the total cost of carbon capture and sequestration (CCS), the remaining 20% being attributable to CO2 compression, pipelining, storage and monitoring. This large cost for the capture portion has, to present, made large scale CCS unvariable; based on data from the IPCC, for instance, for a 700 megawatt (MW) pulverized coal power plant that produces 4 million metric tons of CO2 per year, the capital cost of conventional CO2 capture equipment on a retrofit basis would be nearly $800 million and the annual operating cost and plant energy penalty would be nearly $240 million. As such, there is a need to reduce the costs of the process and develop new and innovative approaches to the problem.
Due to the high costs associated with amine systems, some work has been done based on carbonate solutions. In such carbonate systems, at pH higher than 10, the predominant mechanism for CO2 absorption is:CO2+OH−HCO2−HCO2−+OH−CO22−+H2O
At pH lower than 8 the principal mechanism is:CO2+H2OH2CO2 H2CO2+OH−HCO2−+H2O
The main advantages of carbonate solutions over amine based solutions are higher capacity, higher stability to oxygen and high temperatures and lower energy requirements for desorption. However, such known carbonate solutions are characterized by a low rate of absorption of CO2 which results in large capture equipment and corresponding capital costs.
Another feature of carbonate based solutions is that, as CO2 reacts with the compound, the product may form precipitates. The presence of solids in the absorption solution enables the shift of the chemical reaction equilibria resulting in a constant CO2 pressure when the loading of the solution increases.
Biocatalysts have also been used for CO2 absorption purposes. More specifically, CO2 transformation may be catalyzed by the enzyme carbonic anhydrase as follows:
            CO      2        +                  H        2            ⁢      O        ⁢      ⟷                              ⁢              carbonic        ⁢                                  ⁢        anhydrase            ⁢                            ⁢            H      +        +          HCO      3      -      
Under optimum conditions, the catalyzed turnover rate of this reaction may reach 1×106 molecules/second.
Carbonic anhydrase has been used as an absorption promoter in amine based solutions to increase the rate of CO2 absorption. Indeed, particular focus has been made on amine solutions for use in conjunction with carbonic anhydrase in CO2 capture processes. One reason why amine solutions have been favoured is that they have relatively low ionic strength, which is a property viewed as significant for carbonic anhydrase hydration activity, since high ionic strength could be detrimental to the stability and function of the protein.
There is a need for a technology that overcomes at least some of the disadvantages of the processes and techniques that are already known, and offers an improvement in the field of CO2 capture.